1. Field of the Invention
The present invention relates to an image-recording apparatus in which a recording medium is interposed by a driving roller and nip rollers provided at opposing positions on a transport passage respectively and is transported to an image-recording section, where an image is recorded on the recording medium, arranged on the downstream side of the driving roller and the nip rollers.
2. Description of the Related Art
FIG. 21 shows an arrangement around an image-recording section of a conventional image-recording apparatus. As shown in the drawing, a recording head 91, which discharges the ink while being subjected to the scanning in the widthwise direction of the recording paper, is provided over a transport passage 90 so that the scanning can be performed. A platen 92, which supports the recording paper during the recording, is arranged under the transport passage 90 which is opposed to the recording head 91. A driving roller 93 and nip rollers 95 are provided on the upstream side of the recording head 91 in the transport direction of the recording paper, and a paper discharge roller 94 and spur rollers 96 are provided on the downstream side, while the rollers are disposed as pairs at positions opposed to the transport passage 90. The driving force is transmitted to the driving roller 93 and the paper discharge roller 94 from a driving source (not illustrated) such as a motor via gears or the like. On the other hand, the nip rollers 95 and the spur rollers 96 are arranged so that they are movable upwardly and downwardly. The nip rollers 95 and the spur rollers 96 are urged by springs or the like toward the driving roller 93 or the paper discharge roller 94 respectively to make contact with the driving roller 93 or the paper discharge roller 94 under the pressure.
The recording paper P, which is transported by a transport means from a printing paper tray (not illustrated), is transported onto the platen 92 by being interposed by the driving roller 93 and the nip rollers 95 disposed on the upstream side. When the forward end of the recording paper P arrives at the position under the recording head 91, the scanning of the recording head 91 is started. The ink is discharged from the recording head 91 onto the recording paper P. The driving roller 93 and the nip rollers 95 are driven intermittently by a predetermined line feed amount. Every time when the driving roller 93 and the nip rollers 95 are driven intermittently, the recording head 92 is subjected to the scanning. This operation is repeated, and thus an image is recorded in a predetermined area of the recording paper P transported by every predetermined line feed amount. When the forward end of the recording paper P arrives at the paper discharge roller 94 and the spur rollers 96 disposed on the downstream side, the image recording is performed in such a state that the recording paper P is interposed at the forward end portion thereof by the paper discharge roller 94 and the spur rollers 96, and the recording paper P is interposed at the rearward end portion by the driving roller 93 and the nip rollers 95. When the recording paper P is interposed by the rollers on the upstream side and the downstream side of the recording head 91 respectively as described above, an appropriate tensile force is exerted onto the recording paper P. When the ink droplets, which are discharged from the recording head 91, are landed on the recording paper P, then the solvent of the ink droplets is immediately evaporated, and the shrinkage occurs in the recording paper P. However, the recording paper P on the platen 92 is maintained to be flat by the tensile force as described above. When the recording paper P is further transported, the rearward end of the recording paper P passes out of the driving roller 93 and the nip rollers 95. The recording paper P is transported by the paper discharge roller 94 and the spur rollers 96 disposed on the downstream side. When the image recording is completed, the recording paper P passes out of the paper discharge roller 94 and the spur rollers 96 as well. The recording paper P is discharged to a paper discharge tray (not illustrated).
FIG. 22 shows a plan view illustrating the driving roller 93 and the nip rollers 95 arranged on the upstream side of the recording head 91. As shown in the drawing, the four nip rollers 95, which are disposed at predetermined intervals in the axial direction, are provided in array for one driving roller 93. The shaft of the driving roller 93 is parallel to the shafts of the respective nip rollers 95. As shown by hatched lines in the drawing, the driving roller 93 and the respective nip rollers 95 make contact under the pressure respectively in linear areas disposed in parallel to the shafts to form nip positions N. Therefore, the recording paper P is interposed by the driving roller 93 and the nip rollers 95 at the four linear nip positions N. However, when the recording paper P passes over the respective nip positions N, then the interposing forces, which are exerted by the driving roller 93 and the nip rollers 95, are released at once, and the urging forces of the respective nip rollers 95 are applied to the rearward end of the recording paper P. Accordingly, the recording paper P is pushed out in the transport direction.
On the other hand, as shown in FIG. 21, the spur rollers 96, which are arranged on the downstream side of the recording head 91, make direct contact with the recording surface of the recording paper P immediately after the landing of the ink droplets. Therefore, if the spur rollers 96 are pressed strongly toward the paper discharge roller 94, a problem arises, for example, such that any trace remains on the recording surface. Therefore, any strong pressing force should not be applied to the spur rollers 96. Therefore, it is inevitable that the interposing forces, which are exerted by the paper discharge roller 94 and the spur rollers 96, are made to be weak. When the pushing force is generated when the recording paper P passes over the nip positions N formed by the driving roller 93 and the nip rollers 95 as described above, the pushing force cannot be restrained by the interposing force for the recording paper P exerted by the paper discharge roller 94 and the spur rollers 96. Therefore, a problem of the so called “overfeed” arises such that the recording paper P is transported by an amount not less than the predetermined line feed amount by the pushing force as if the recording paper P slides out the nip positions formed by the paper discharge roller 94 and the spur rollers 96. If the overfeed occurs, a problem arises such that the recording position is deviated in the secondary scanning direction, and any unevenness and/or white blank appears in the recorded image.
A structure as shown in FIG. 23A is known as a countermeasure to solve the problems as described above (see Japanese Patent Application Laid-open No. 2002-226077). In this structure, a driving roller 97 having a large diameter is provided on the upstream side of the recording head 91. On the other hand, two nip rollers 98, each of which has a diameter smaller than that of the driving roller 97 and each of which is formed to have a large diameter section and a small diameter section in a tapered form, are provided as a pair. The pair of nip rollers 98 are arranged coaxially so that the respective large diameter sections are positioned at the both ends to provide one roller set 99. The four roller sets 99 are provided in array at predetermined intervals in the axial direction of the driving roller 97. The shaft of each of the roller sets 99 is arranged by being inclined by a predetermined angle with respect to the shaft of the driving roller 97. The angles of inclination of the two roller sets 99 arranged at the both ends are larger than the angles of inclination of the two roller sets 99 arranged at the central portions of the driving roller 97. Accordingly, the nips at the nip positions N between the driving roller 97 and the respective nip rollers 98 are formed as points, and the positions in the transport direction are deviated from each other. Therefore, the timings, at which the recording paper P passes over the respective nip positions N, are deviated from one another. The recording paper P, which is interposed by the driving roller 97 and the nip rollers 98, is released in a stepwise manner. The overfeed of the recording paper P is suppressed.
According to Japanese Patent Application Laid-open No. 2002-226077, it is disclosed that the nip rollers and the driving roller are arranged in the relationship as shown in FIG. 23B (corresponding to FIG. 4(b) of Japanese Patent Application Laid-open No. 2002-226077). The nip roller (pinch roller) 98 is rotatably attached to a rotary shaft provided on a holder (not illustrated). As described above, the rotary shaft of the nip roller 98 is attached while being inclined by the predetermined angle with respect to the rotary shaft of the driving roller 97. The holder is urged by a spring, and thus the nip roller 98 is pressed against the upper portion of the driving roller 97. The large diameter sections 98a, 98d of the nip roller 98 make contact with the driving roller 97 under the pressure to form the nips. In this situation, both of the nip position formed by the large diameter section 98a and the driving roller 97 and the nip position formed by the large diameter section 98d and the driving roller 97 are positioned on the lower side and on the downstream side in the transport direction as compared with the uppermost end of the driving roller 97. As shown in FIG. 23B, the nip position formed by the large diameter section 98d and the driving roller 97 is positioned on the further downstream side in the transport direction as compared with the nip position formed by the large diameter section 98a and the driving roller 97. Therefore, in the case of the structure described in Japanese Patent Application Laid-open No. 2002-226077, the recording medium is disengaged from the nip positions in a stepwise manner while the rearward end passes over the uppermost end portion of the driving roller to move downwardly in a state in which the forward end of the recording medium is transported on the transport passage while being interposed by the roller set disposed on the downstream side of the image-recording section. That is, the position, at which the contact of the recording medium with the nip roller and the driving roller comes to the end, is coincident with the nip position formed by the large diameter section 98d and the driving roller 97. Therefore, the rearward end of the recording medium becomes immediately in a free state after being disengaged from the nip position in a state of being urged in the downward direction. Therefore, the rearward end of the recording medium tends to float upwardly. When the rearward end floats upwardly, then the flatness of the entire recording medium is deteriorated, and any uniform spacing distance is not formed between the recording medium and the recording head in the image-recording section. Therefore, a problem arises such that the recording quality is deteriorated.
In Japanese Patent Application Laid-open No. 2002-226077, the nip positions, which are formed by the large diameter sections of the nip roller and the driving roller, are deviated in the transport direction, and the recording medium is transported while being positionally adjusted on one end edge side in the transport direction. Therefore, when any recording medium, which has a non-standard size in the widthwise direction, is interposed, the nip positions are not symmetrical in the widthwise direction of the recording medium, resulting in the deviation, in the transport direction, of the nip positions and the contact start positions of the driving roller and the nip roller with respect to the recording medium. Therefore, there has been such a possibility that the recording medium may be transported while being inclined with respect to the transport direction.
As described above, the outer diameter of each of the nip rollers 98 is changed in the tapered form in the axial direction, and the shaft or the axis thereof is inclined with respect to the driving roller 97. Further, the nip roller 98 is rotatably attached to the rotary shaft provided in the holder urged by the spring, which is pressed by the driving roller 97. It is considered that the nip positions N, at which the respective nip rollers 98 of one roller set 99 make contact with the driving roller 97 under the pressure, are not the left-right symmetrical positions in some cases with respect to the axis perpendicular to the shaft of the roller set 99 depending on the positions of the respective nip rollers 98 with respect to the driving roller 97. In such a situation, the speeds of rotation of the nip rollers 98 driven by the rotation of the driving roller 97 are not identical on the left and right sides, because the diameters of the nip rollers 98 at the nip positions are different between the left and right rollers. In order to respond to the situation as described above, it is necessary that the left and right nip rollers 98 should be rotated independently. When the left and right nip rollers 98 can be freely rotated with respect to the shaft of the roller set 99 as in Japanese Patent Application Laid-open No. 2002-226077, the respective nip rollers 98 are movable in the axial direction as well, and there is such a possibility that the spacing distance between the nip positions N of one roller set 99 may be varied. In this case, a problem is considered to arise such that it is difficult to correctly establish the respective nip positions N in the transport direction. On the other hand, in order that the left and right nip rollers 98 are fixed to the shaft of the roller set 99 to obtain the same rotation on the left and right sides, it is necessary that any constant tapered shape is provided so that the outer diameters of the respective nip rollers 98 are in the left-right symmetrical in each roller set 99, and the positioning should be effected with respect to the driving roller 97 so that the rotation is identical on the left and right sides. Consequently, a problem arises such that it is complicated to produce the nip rollers 98 and assemble the driving roller 97 and the respective roller sets 99.
When the nip roller is rotatably supported by the rotary shaft (support shaft) provided on the holder (pinch roller holder), and the holder is pressed by the spring toward the driving roller as in Japanese Patent Application Laid-open No. 2002-226077, then the reaction force of the pressing force acts on the bearing section of the support shaft. However, in general, the holder and the nip roller are made of synthetic resin materials such as ABS, and the support shaft of the nip roller is made of metal (steel material). Therefore, the following problem arises. That is, when the nip roller and the support shaft are rotated in an integrated manner during the transport of the recording medium, then the bearing portion made of synthetic resin is cut or scraped, and backlash is caused. If the bearing section is greatly cut or scraped, then the axial center position of the support shaft is deviated away from the driving roller, and the holder surface (transport surface) relatively abuts against the outer circumferential surface of the driving roller. Therefore, this causes a paper jam. When the driving roller is made of a hard material, the surface of the holder is cut or scraped.
Even when a lubricant such as grease is applied to the bearing section in order to solve the problem as described above, the paper powder acts as a polishing material, when the paper powder originating from the recording medium is mixed into the grease. A problem arises such that the bearing section is increasingly cut or scraped. Further, the following problem also arises. That is, when the grease is applied to the bearing section, for example, the grease is liquefied to adhere and stain the recording medium, when the environmental temperature is raised.
It is also conceived that a part of the outer circumference at the end of the support shaft is formed to have a D-shaped cross section in order to solve the problem as described above. However, when the diameter of the support shaft is small, it is difficult to perform the processing or the machining therefor. Further, it is necessary that a D-shaped hole should be also formed for the bearing section. Problems arise such that the production cost is also expensive, and the operation for incorporating the support shaft into the bearing section is troublesome as well.